Optical storage media are media in which data are stored in an optically readable manner, for example by means of a laser and a photo-detector being integrated within a pickup. The photo-detector is used for detecting the reflected light of the laser beam when reading data from the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store up to 50 GB on a dual layer disc. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2T to 8T and 9T is used, where T is the channel bit length, and which corresponds with a minimum mark length of 138-160 nm. The re-writable BD-RE disc is based on a phase change technology comprising a phase change layer, which uses for example a compound of AgInSbTe or GeSbTe. Further information about the Blu-Ray disc system is available for example from the Blu-Ray group via internet: www.blu-raydisc.com.
New optical storage media with a super-resolution structure offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-Ray disc. This is possible by including a nonlinear layer, which is placed above the data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The nonlinear layer can be understood as a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Further, semiconductor materials can be used as a nonlinear layer, e.g. InSb, which show a higher reflectivity in the center part of the focused laser beam, and which center reflectivity is dependent on the pit structure of the corresponding data layer. Therefore, the super-resolution effect allows to record and read data stored in marks of an optical disc, which have a size below the diffraction limit of a corresponding optical pickup.
In the articles Hyot et al, “Phase change materials and Super-RENS”, E*PCOS 05, Technical Digest, Cambridge, 2005, and Pichon et al, “Multiphysics Simulation of Super-Resolution BD ROM Optical Disk Readout” 2006 IEEE, 0-7803-9494-1/06, PP 206-208, a semi-conducting nonlinear layer is proposed in which a local change of the refractive index can be obtained through photo generation of free carriers. A thermal description is given to provide information on temperature distribution of a data layer during readout.
It has been shown that the optimum thickness of the InSb nonlinear layer itself is about 20 nm due to the microcrystalline structure, as described in Hyot et al., “Super-Resolution ROM disk with a semi-conductive InSb active layer: influence of the crystalline microstructure”, ISOM Tech. Dig., 2007, p. 12. Including some tolerance, the optimum thickness can be expected to be in a range of 15 to 25 nm. For thicker InSb layers, it is assumed that the crystalline microstructure is increasing when heating with a laser and therefore the super-resolution effect is reduced. As a consequence, a thicker InSb layer cannot be used to improve the reflectivity change.